The present invention relates to devices and methods for distraction and stabilization of tissue surfaces, and most particularly for stabilization of the intervertebral disc space in interbody fusion applications.
The number of spinal surgeries to correct the causes of low back pain has steadily increased over the last several years. Most often, low back pain originates from damage or defects in the spinal disc between adjacent vertebrae. The disc can be herniated or can be suffering from a variety of degenerative conditions, so that in either case the anatomical function of the spinal disc is disrupted. The most prevalent surgical treatment for these types of conditions has been to fuse the two vertebrae surrounding the affected disc. In most cases, the entire disc will be removed, except for the annulus, by way of a discectomy procedure. Since the damaged disc material has been removed, something must be positioned within the intra-discal space, otherwise the space may collapse resulting in damage to the nerves extending along the spinal column.
In order to prevent this disc space collapse, the intra-discal space has been filled with bone or a bone substitute in order to fuse the two adjacent vertebrae together. In early techniques, bone material was simply disposed between the adjacent vertebrae, typically at the posterior aspect of the vertebrae, and the spinal column was stabilized by way of a plate or a rod spanning the affected vertebrae. With this technique, once fusion has occurred the hardware used to maintain the stability of the segment became superfluous. Moreover, the surgical procedures necessary to implant a rod or plate to stabilize the level during fusion were frequently lengthy and involved.
It was therefore determined that a more optimum solution to the stabilization of an excised disc space is to fuse the vertebrae between their respective end plates, most optimally without the need for anterior or posterior plating. There have been an extensive number of attempts to develop an acceptable intra-discal implant that could be used to replace a damaged disc and yet maintain the stability of the disc interspace between the adjacent vertebrae, at least until complete arthrodesis is achieved. These “interbody fusion devices” have taken many forms, but many have had difficulty in achieving fusion, at least without the aid of some additional stabilizing device, such as a rod or plate. Moreover, some of these devices are not structurally strong enough to support the heavy loads and bending moments applied at the most frequently fused vertebral levels, namely those in the lower lumbar spine.
The interbody fusion devices (IBFDs) that have overcome these difficulties are typically bulky, at least with respect to the intervertebral space. In particular, these devices have been configured to completely fill the space and to restore the normal spinal anatomy at the instrumented level. One drawback of this approach is that the implant device is not exactly sized to the anatomy of the particular patient, thus typically requiring pre-distraction of opposed vertebrae in order to increase the disc space for device implantation. While a collection of differently sized IBFDs can be provided, it is unwieldy and impractical to provide an IBFD sized for every intervertebral disc space height.
Another drawback of these prior devices is that the surgical insertion site must be at least as big as the IBFD. Minimally invasive and working channel surgical techniques have been recently developed that have significantly reduced the surgical invasion, but even more improvement is needed. One solution to these drawbacks was presented in U.S. Pat. No. 6,595,998 (the '998 Patent), entitled “Tissue Distraction Device”, which issued on Jul. 22, 2003, to the assignee of the present invention. The '998 Patent discloses sequentially introducing a series of wafers into the space (whether inter- or intra-vertebral) using a percutaneous introducer. In certain embodiments, the wafers included features that allowed adjacent wafers to interlock to some degree along the longitudinal axis of the wafers. The disclosure of the '998 Patent is incorporated herein by reference, particularly as it pertains to the interlocking features of the wafers and the percutaneous introducer.
In an improvement on the wafer concept in the '998 Patent, an expandable distraction device was disclosed in co-owned pending application U.S. application Ser. No. 10/813,819 (the '819 Application), which was filed on Mar. 31, 2004, and published as U.S. Patent Application Publication No. 2005/0187559 on Aug. 25, 2005. The disclosure of the '819 Application is incorporated herein by reference. The expandable distraction device disclosed in the '819 Application includes a plurality of wafers that are successively inserted to form a stack of wafers in a column. The wafers are configured so that a newly inserted wafer lifts the stack of previously inserted wafers, including the superior endplate, until the space has been distracted to a desired height.
A further improvement is disclosed in co-owned pending application U.S. application Ser. No. 11/211,346 (the '346 Application), which was filed on Aug. 25, 2005, and was published as U.S. Patent Application Publication No. 2006/0058807 on Mar. 16, 2006. The disclosure of the '346 Application is incorporated herein by reference. The '346 Application discloses a wafer insertion apparatus 50 that includes a wafer track 52 configured at one end to releasably engage the inferior endplate of the expandable device 10, as depicted in FIG. 1. The other end of the wafer track is connected to a gun 51 that supports a cartridge of wafers 54 and includes a trigger-operated mechanism 53 for extracting a wafer from the cartridge and advancing it along the wafer track into the wafer cavity between the superior and inferior endplates. The wafer insertion apparatus initially supports the expandable device in situ and includes a release plate operable to separate the wafer track from the expandable device when wafer insertion is complete.
The wafer insertion apparatus disclosed in the '346 Application utilizes a series of posts as shown for example in FIGS. 44-45 thereof formed in the wafer cavity defined by the inferior endplate. The posts are engaged by an insertion plate that forms part of the wafer track so that the track can support the expandable distraction device in situ during wafer insertion. A release plate severs the posts to allow the wafer track to disengage the inferior endplate for removal of the wafer insertion apparatus.
The wafers disclosed in the '346 Application include features that facilitate interlocking between adjacent wafers. Thus, as illustrated for example in FIGS. 28-29 and FIGS. 35-36 of the '346 Application, the wafers include resiliently deflectable features that deflect and lock upon longitudinal insertion of a new wafer underneath a previously inserted wafer.
In preferred uses of the expandable devices described above, it is contemplated that bone promotion filler such as osteoinductive or osteoconductive material may be integrated around, and in some cases into, the stack of wafers forming the distraction device. Ideally, once fusion occurs the entire space is rigid, as if the entire space is bone. In an interbody fusion procedure, the vertebrae adjacent the affected disc space are fused together so that the motion segment is eliminated at the disc level. The distracted space is subjected to significant loads, even when efforts are made to immobilize the spine around the affected vertebral level. While the compressive loads along the length of the spine are readily borne by the expanded distraction device and associated wafer stack, transverse loads and most particularly torsion loads must also be withstood.
Consequently, there remains a need for an expandable distraction device that can endure the significant spinal loads and maintain suitable structural integrity, at least until complete fusion can be achieved.